Flow resistance of flexible vegetation in real-scale drainage channels

The definition of simple and accurate methods to estimate flow resistance in vegetated channels is still a challenging issue in soil bioengineering practices and programming riparian vegetation management to control channel conveyance capacity, sediment deposition, and flooding propensity. In this paper, measurements collected by Errico et al. (2018, 2019) in drainage channels colonized by common reed (Phragmites australis) were used to study the effect of flexible vegetation and its management in flow resistance estimate. At first, a theoretical flow resistance equation, obtained applying dimensional analysis and incomplete self-similarity condition for the velocity distribution of an open channel flow, was briefly summarized. Then, this flow resistance equation was calibrated and tested by open-field hydraulic experiments carried out by Errico et al. (2018, 2019) at the real scale of existing vegetated drainage channels. In particular, the Gamma function of the power velocity profile was empirically related to the slope energy and the flow Froude number by using the available measurements. Taking into account the hydrological regime of the flow in the investigated channels, the original data set was divided into two sub-data sets (calibrating and testing data set) exploring the same range of measured discharges. The calibration and testing of the flow resistance equation were carried out without distinguishing measurements corresponding to different vegetation conditions (full-vegetated, half-vegetated, non-vegetated, central vegetation cut, extensive vegetation cut). The analysis demonstrated that the theoretical flow resistance equation allows an accurate estimate of the Darcy-Weisbach friction factor which is characterized by errors that are always less than 10% and less than or equal to 5% for 90.9% of the investigated cases. The finding of this study also allowed to evaluate the effects of different vegetation management scenarios on flow resistance.